In recent years, an internal structure of an object is imaged using X-ray beams in various fields, such as manufacturing field sites of things, such as semiconductors, construction field sites such as for pipelines, baggage inspection in airports, or medical field sites. In particular, in the field sites of medical research and treatment, what is essential now as one of diagnostic methods is to acquire an internal tomographic image of an object being examined using medical modalities, such as an X-ray imaging apparatus or an X-ray CT.
In conducting an X-ray diagnosis in the medical field sites, it is very important to know how the imaging portion of a patient, as a target of diagnosis, has changed in terms of time. As a matter of course, materials deteriorate with time and accordingly observation of such temporal changes is not limited to patients.
As an algorithm for acquiring information on such temporal changes, Non-patent Document 1 discloses a subtraction method based on a phase-limited correlation method. When the subtraction method is performed, two two- or three-dimensional images are picked up from one portion of an object being examined at different time points. Of the two images, one is used to designate two or three specific positions thereon. Then, positions corresponding to the designated positions are specified on the other two- or three-dimensional image by calculating a strongest phase correlation. In order to mutually align the designated and specified positions of both images, a motion vector (indicating magnification/reduction, rotation or parallel translation) is calculated for each position. Using the motion vectors, one image is aligned with the other image and subtraction between both images is acquired on a pixel basis. Thus, temporal changes of the imaging portion of the object being examined can be acquired from both images.
The subtraction method using the phase-limited correlation method is already used in a device related, for example, to an application filed by the applicant of the present invention in the past (see Patent Document 1).